The present invention relates to a fluidized bed firing system with a combustion chamber having a bottom which is penetrated by coal nozzles for introducing a coal/air mixture and by air nozzles for injecting combustion air into the fluidized bed.
By applying special nozzle forms for such fluidized bed firing systems, the attempt is made to distribute the fuel as uniformly as possible in the fluidized bed. The granular fuel thus admitted shall intimately mix with the bed material immediately after having entered into the fluidized bed. Strong mixing is promoted by the rising of bubbles within the fluidized bed.
When the fuel admitted through the coal nozzles, consists of a grain fraction with an increased amount of fine particles and when it is pneumatically introduced into the fluidized bed, the small particles will be entrained by the main gas stream in the main flow direction. In the region of the bubbles, this main flow direction of the solids is given by the bubble rising direction. The bubbles have consequently the twofold disadvantage: in their border zone and in their cloud they transport unburnt fuel particles through the fluidized bed into the freeboard above the fluidized bed; in their border zone they have, moreover, a poorer exchange of matter than within the homogeneous area of the fluidized bed.